It is well known that C-MOS devices require power supplies for their proper operation. It is also well recognized that often times there is more than one positive voltage applied to the circuit at any one time. It is also true that, in those types of devices, more than one positive voltage can be applied at any time, those two voltages can be at different levels. More particularly, in certain C-MOS technologies it is important that the voltage applied to the substrate be the higher of any two levels of voltage applied thereto. If the higher voltage is not applied to the substrate then the P-N junction formed by the N-substrate and a P-diffusion will be forward biased, and the resulting current may cause the device's circuitry to latch up. The latch up condition, if maintained on the C-MOS device, could draw excessive current from the power supply associated with the device and also potentially destroy the integrated circuit employed in the C-MOS circuitry.
In U.S. Pat. No. 4,473,758 issued on Sept. 25, 1984 in the name of Robert C. Huntington, a substrate bias control circuit and method are disclosed. This patent describes a substrate bias voltage control circuit that insures that a substrate has a supply of voltage applied thereto which includes a means of providing sources of bias and supply voltage to the substrate. The apparatus as described in the above-mentioned patent also provides a means for coupling the bias voltage to the substrate when the bias voltage is present and then if the bias voltage is not present it provides for means for coupling the supply voltage to the substrate. Although this device works properly for its intended purpose, it does not address the problem of insuring that the highest voltage being applied to the device is in turn also applied to the substrate. It, in effect, selects a bias voltage whenever it is present irrespective of whether it is higher or lower than the supply voltage applied thereto.
Accordingly, what is needed is apparatus and methods for the C-MOS device preventing a higher voltage being applied to the device than that applied to the substrate. What is also needed is circuitry to insure that the protection to the device is provided in a reliable manner.